As a tobacco filter which removes tars from the tobacco smoke and insures a satisfactory smoking quality, a filter plug prepared by shaping a tow (fiber bundle) of cellulose acetate fiber with a plasticizer such as triacetin is known. In this filter, however, the constituent filaments have been partly fused together by the plasticizer so that when it is discarded after smoking, it takes a long time for the filter plug to disintegrate itself in the environment, thus adding to the pollution problem.
Meanwhile, a tobacco filter made of a creped paper manufactured from wood pulp and a tobacco filter made from a regenerated cellulose fiber are also known. Compared with a filter plug comprising a cellulose acetate fiber, these filters are slightly more wet-disintegratable and, thus, of somewhat lower pollution potential. However, in these filters, not only the aroma and palatability of tobacco smoke are sacrificed but the efficiency of selective elimination of phenols which is essential to tobacco filters can hardly be expected. Moreover, at the same pressure loss, the firmness or hardness of these filters is lower than that of the cellulose acetate filter.
Japanese Patent Application Laid-open No. 96208/1977 (JP-A-52-96208) discloses a sheet consisting of an acetylcellulose pulp prepared in a specified manner and short staples of a thermoplastic resin. However, because this sheet is manufactured by mix-webbing the pulp and short staples and heating the resulting paper under pressure, it is high in tensile strength and elongation after immersion in water as well as in water resistance and very low in disintegratability.
Japanese Patent Application Laid-open No. 45468/1978 (JP-A-53-45468) corresponding to U.S patent application Ser. No. 730039 discloses a filter material comprising a nonwoven sheet containing 5 to 35 weight % of fine cellulose ester fibrils with a large surface area and 65 to 95 weight % of cellulose ester short staples. Furthermore, this prior art literature mentions that wood pulp may be incorporated in this mixture of cellulose ester fibrils and cellulose ester short staples. However, because cellulose esters can hardly be processed into fine fibrils, a special technique is required for providing the fine fibrils with a large surface area. Moreover, the disintegratability of this filter material is not sufficiently high so that the risk of pollution is substantial.
Furthermore, a tobacco filter material in sheet form is required to retain a high strength during dry handling but, then, its wet disintegratability is low. By the same token, a sheet material providing for a high degree of wet disintegratability shows only a low strength even in dry handling condition. Thus, the high dry sheet strength and high wet disintegratability can hardly be reconciled.